Schottky barrier diode and method of making the same

ABSTRACT

A power Schottky rectifier device having pluralities of trenches are disclosed. The Schottky barrier rectifier device includes field oxide region having p-doped region formed thereunder to avoid premature of breakdown voltage and having a plurality of trenches formed in between field oxide regions to increase the anode area thereto increase forward current capacity or to shrinkage the planar area for driving the same current capacity. Furthermore, the trenches have rounded corners to alleviate current leakage and LOCOS region in the active region to relief stress during the bonding process. The processes for power Schottky barrier rectifier device including termination region formation need only three masks and thus can gain the benefits of cost down.

FIELD OF THE INVENTION

The present invention relates to a semiconductor process, specifically,to a Schottky barrier power rectifier having buried p+ layers underLOCOS structure to reduce reverse leakage current and improve breakdownvoltage, and pluralities of trenches in between them to increase theactive area resulting in higher forward current capacity.

BACKGROUND OF THE INVENTION

Schottky diode is an important power device and used extensively asoutput rectifiers in switching-mode power supplies and in otherhigh-speed power switching applications, such as motor drives, switchingof communication device, industry automation and electronic automationand so on. The power devices are usually required characteristics ofcarrying large forward current, high reverse-biased blocking voltage,such as above 100 volt, and minimizing the reverse-biased leakagecurrent.

A number of power rectifiers have been used to provide high current andreverse blocking characteristics. An exemplary method to form a Schottkybarrier diode is disclosed by Chang et al in U.S. Pat. No. 6,404,033.The processes are shown in FIG. 1A to FIG. 1C. Referring to FIG. 1A, asemiconductor substrate having an n+ doped layer 10 and an n−drift layer12 extended to a first surface 13 is prepared. A field oxide layer 14 isthen formed on the first surface 13. Afterward, the field oxide layer 14is patterned to define positions of guard ring 22 at the terminationregion. Guard ring regions 22 are then buried into n−drift layer 12 bydouble implants with B⁺ and BF₂ ⁺ as conductive impurities. Thereafter,a thermal anneal process is then performed to drive in and activate theimpurities. Thereafter, a second photoresist pattern 24 is then coatedon the resultant surface to define an anode region. The results areshown in FIG. 1B.

Referring to FIG. 1C, a wet etch is then performed to remove thoseexposed field oxide layer 14. After stripping away the photoresistpattern 24, another photoresist pattern 28 having openings is formed onthe resultant surface to define trenches at the active region. Anetching step is then performed to recess the drift layer 12 using thephotoresist pattern 28 as a mask. Another B⁺ or BF₂ ⁺ ion implant isthen carried out to form p type region 30 buried into trench bottom.

Referring to FIG. 1D, the photoresist pattern 28 is removed. Then, aSchottky barrier metal layer 32 is formed on the resultant surface.Thereafter, a top metal layer formation is followed. A forth photoresist(not shown) and an etch steps are then performed to define the topelectrode 36. After the layers formed on the backside surface duringforgoing step are removed, a metal layer 60 is then formed, which isused as a bottom electrode 34.

Although the Schottky barrier rectifier disclosed in U.S. Pat. No.6,404,033 having pluralities of trenches to increase the surface areathereto increases forward current capacity and having buried p layers 30at the bottom of the trenches to form p-n junction regions to increasebreakdown voltage. However, it requires a complex processes at leastfour to six masks. And also, the buried p-n junctions will introducemany minority carriers when device is under forward bias, which willresult in a larger reverse recovery time than the typical Schottkybarrier rectifier. The object of the present method is to improve thebreakdown voltage and enhance the forward current capacity and simplifythe manufacturing processes.

SUMMARY OF THE INVENTION

A power Schottky rectifier device and method of making the same aredisclosed. The Schottky rectifier device includes field oxide regionhaving p doped region formed thereunder to avoid premature of breakdownvoltage and having a plurality of trenches formed in between field oxideregions to increase the surface area so as to enhance forward currentcapacity. Furthermore, the corner of trench has been rounded toalleviate the reverse-biased leakage current. The present method ofSchottky barrier rectifier comprises the following steps: firstly, an n+doped substrate formed with an n−drift layer, and a pad oxide layer isprovided. Subsequently, a nitride layer is formed on the pad oxidelayer. The active area is then defined by using the photolithography anddry etching processes. After p-type impurities implant (e.g., boron orBF₂ ⁺ ion implant), a thermal oxidation is then performed to form fieldoxide region in the active region and termination oxide region at theperimeter of the substrate. A second patterning process is thenimplemented to form a plurality of trenches in between each oxideregions. Thereafter, another thermal oxidation process is carried out torecover etching damages and round the corners in each trench. Afterremoving the oxide layer, a metallization process is performed to formmetal (or silicide) layer on the surfaces of the trenches and the mesas.Next, a top metal layer is formed on the front surface of the substrate.The top metal layer is then patterned to defined anode electrode. Then,the backside metallization is formed to serve as cathode electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

FIGS. 1A to 1D show steps of forming a conventional Schottky barrierdiode with a plurality of trenches and p-region at the bottom of eachtrench and p+ guard ring structure at the termination in accordance withprior art.

FIG. 2 is a cross-sectional view of forming nitride mask pattern layeron the oxide layer and forming p-type impurities implant region into n−epi layer in accordance with the present invention.

FIG. 3 is a cross-sectional view of performing thermal oxidation to formfield oxide regions, and termination region, as well as to extend the player region in accordance with the present invention.

FIG. 4 is a cross-sectional view of forming a plurality of trenches inbetween of field oxide regions and in between field oxide region andtermination region

FIG. 5 is a cross-sectional view of forming thermal oxide layer to roundthe trench corners.

FIG. 6 is a cross-sectional view of forming metal (or silicide) on thesurfaces of trenches and mesas and forming a patterned anode electrodeon the front surface and a cathode electrode on the back side surface ofthe substrate.

FIG. 7 is a synoptic layout showing a plurality of trenches distributedin the spacing of field oxide regions at the active area and thetermination region.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As depicted in the forgoing background of the invention, to form a powerrectifier device and its termination structure using the conventionaltechnique requires at least four to six photo masks. The presentinvention can simplify the processes by using only three photo masks.The detailed descriptions are as follows:

Firstly, an n+ doped substrate 100 formed with an n−drift layer 105, anda pad oxide layer 110 is provided. To define active region, referring toFIG. 2, a nitride layer 120 is formed on the oxide layer 110. Aphotoresist pattern 125 coated on the nitride layer 120 to define activeregion is then followed. Subsequently, an etching back step is performedto etch the nitride layer 120 and the oxide layer 110 by using thephotoresist pattern 125 as a mask.

After active region definition, a p-type impurity implantation process,for example, implants B⁺ and BF₂ ⁺ ions into the n− epi layer 105 toform a p region 135 is then successively performed. The dosage and theimplant energy are about 5×10¹⁰–5×10¹⁴/cm² and 10–1000 keV for boronions and about 5×10¹¹–5×10¹⁵/cm² and 30–300 keV for BF₂ ⁺ ions.

After ion implantation, the photoresist pattern 125 is stripped away anda thermal oxidation process is followed by using the nitride layer 120as a mask, as is shown in FIG. 3. During the thermal oxidation process,a pair of thick field oxide regions 140 buried into the active region ofthe substrate and thick oxide termination regions 140A buried into theperimeter of the substrate are grown by using the nitride layer 120 as amask. In addition, the ions in the p regions 135, are driven in bothlaterally and longitudinally into n− epi layer 105 and results inextending the regions thereof.

In a preferred embodiment, the width W of the mesa region 150A inbetween two field oxide regions 140 and in between the field oxideregion 140 and termination is between about 10–1000 μm for field oxideregion having 0.3–2 μm in thickness and the p/n junction 135/105 havinga depth D1 of about 0.3–3 μm from the surface of the mesa region 150A.

Referring to FIG. 4, the nitride layer 120 is removed firstly. Aphotoresist pattern 142 formation is then followed to define a pluralityof trenches. Subsequently, an etching process is performed to recess then− epi layer 105. Preferably, each resulted trench 145 has a width W1and depth D2 of about 0.5 to 5 μm and 0.1 to 5 μm, respectively. Thesub-mesa 160 width W2 in between two trenches 145 is about 0.5 to 5 μm.

Thereafter, the photoresist pattern 142 is stripped away, and a thermaloxidation process forming an oxide layer 146 is then performed. Theoxidation process is performed to recover etching damages and make thetrench corner rounding so as to alleviate the problem of currentleakage. The resulted structure is shown in FIG. 5.

Please refer to FIG. 6, a wet etch is followed to remove the oxide layer146. Afterward, a barrier metal layer deposited on the front surface isthen followed. The material of the barrier metal, for example, Al, AlCu,AlSiCu, Ti, Ni, Cr, Mo, Pt, Zr, W etc., can be acted as candidates.After a metallization process to form a Schottky barrier layer 170 onthe mesa region 160 and all of the sidewalls and bottoms of trenches145. The unreacted metal layer on both field oxide region 140 andtermination region 140A is then removed by a wet etch. Afterward, a topmetal layer 180 is deposited on the Schottky barrier layer 170 andcovered the field oxide regions 140 and the termination region 140A. Thematerial of the top metal layer 180 is chosen, for example, Al, AlCu,AlSiCu, Ti/Ni/Ag etc. Subsequently, the top metal layer 180 is patternedto serve as anode which includes extension portions of the top metallayer 180 on the termination regions 140A. A backside metal layer 190 isformed as a cathode electrode.

FIG. 7 shows a synoptic layout of the devices in accordance with thepresent invention.

The Benefits of this Invention are:

-   (1) The processes provided are much simpler than the conventional    methods. The method according to the present invention demands only    three photo masks: the first one is for active regions definition,    the second for pluralities of trenches definition, and the three for    top metal electrode definition.-   (2) The field oxide regions in the active region of the substrate    can serve as a buffer layer for stress relief during the bonding    process. In addition, the buried p+ layer 130/135 and field oxide    regions 140 can also improve the breakdown voltage.-   (3) The termination field oxide regions 140A are broad and flatted    and thus the bending regions of the depletion boundary are    anticipated to be far away from the active region than the    conventional device.-   (4) The pluralities of trenches in between field oxide region can    increases significantly surface area to enhance forward current    capacity.

As is understood by a person skilled in the art, the foregoing preferredembodiment of the present invention is an illustration of the presentinvention rather than limiting thereon. It is intended to cover variousmodifications and similar arrangements included within the spirit andscope of the appended claims, the scope of which should be accorded thebroadest interpretation so as to encompass all such modifications andsimilar structure.

What is claimed is:
 1. A power rectifier device, comprising: an n−driftlayer formed on an n+ substrate; a cathode metal layer formed on asurface of said n+ substrate opposite said n−drift layer; a pair offield oxide regions formed into said n−drift layer, and said field oxideregions separated by a first mesa; a pair of termination regions spaced,respectively, said pair of field oxide regions with a second mesa; saidfirst mesa and said second mesa having trenches formed into said n−driftlayer; four p-type doped regions, respectively, right beneath each ofsaid termination regions and said field oxide regions; a barrier metallayer formed on sidewalls and bottom of said trenches, and formed onremnant portions of said first mesa and said second mesa; and a topmetal layer acted as an anode electrode formed on said barrier metallayer, said field oxide regions and extended to cover a portion of saidtermination regions.
 2. The power rectifier device according to claim 1wherein said barrier metal layer is formed from the group of Al, AlCu,AlSiCu, Ti, Ni, Cr. Mo, Pt, Zr, and W, Ti/TiN, etc.
 3. The powerrectifier device according to claim 1 wherein said top metal layer isformed of stack layers of Al, AlCu, AlSiCu or Ti/Ni/Ag.
 4. A powerrectifier device, comprising: an n−drift layer formed on an n+substrate; a cathode metal layer formed on a surface of said n+substrate opposite said n−drift layer; a pair of field oxide regionsformed into said n−drift layer, and said field oxide regions separatedby a first mesa; a pair of termination regions spaced apart, said pairof field oxide regions with a second mesa; said first mesa and saidsecond mesa having trenches formed into said n−drift layer; four p-typedoped regions, one of said termination regions and said field oxideregions are located on each of the four p-type doped regions; a barriermetal layer formed on sidewalls and bottom of said trenches, and formedon remnant portions of said first mesa and said second mesa; and a topmetal layer acted as an anode electrode formed on said barrier metallayer, said field oxide regions and extended to cover a portion of saidtermination regions.